Greenhouse Use of Organic Fertilizers and Composts – 2
Floriculture Research Report 20-04
Nutrient Analysis of Selected
Commercial Organic Fertilizers for
Greenhouse Lettuce Production
Robert G. Anderson and L. Stephanie Schmidt, Department of Horticulture
Introduction
The market for organic produce continues to expand (Thompson, 2000). Kentucky has
more than 75 acres of greenhouses with modified pond or tank hydroponic beds for "float"
tobacco transplant production. The
development of a certified organic
greenhouse production system for
lettuces and greens could allow Kentucky
tobacco farmers access to a new market
for their facilities as the tobacco market
changes. In this study, commercial
organic fertilizers were used to grow
‘Ostinata’ Bibb and ‘Red Sails’ leaf
lettuce in a pond, tank, or "float"
production system. Plant growth was
evaluated, and the fertilizer solutions
were analyzed for nutrient amounts and
Lettuce was grown in 1 oz. soufflé cups placed into
compared to recommended standards for
a polystyrene board floating on the nutrient solution.
inorganic fertilizers (Muckle, 1993,
Thompson et al., 1998).
Materials and Methods
In this study, nine 12-square-foot wooden hydroponic ponds or tanks were built in three
rows of three on one side of a 30-foot x 60-foot naturally ventilated sidewall plastic greenhouse.
Tanks were lined with black polyethylene and filled with water to a depth of 6 inches to make a
tank volume of approximately 38 gallons. Electric water pumps were placed in each tank to
oxygenate the water; previous work demonstrated that oxygen levels would be maintained at 4 to
6 ppm with this procedure. Holes (35) were cut in eighteen 36-inch x 22-inch x 1-inch polystyrene
sheets. The holes were 1.5 inches in diameter and spaced 5 inches x 6 inches. Lettuce plants were
grown in 1-ounce plastic soufflé cups (Solo Cup Company, Urbana, IL) that had holes drilled in
the bottom.
Inorganic fertilizer (Peter's 20-10-20, Scotts, Marysville, Ohio) or one of three
commercially available, water-soluble organic fertilizers (Peaceful Valley Farm Supply, Grass
Valley, California) was added to the water in each tank. Algamin, 0.2N-0P-3.3K, (18 percent cold
processed kelp, Ascophyllum nodulosum from Norway) was applied at the label rate and three
Kentucky Agricultural Experiment Station • University of Kentucky • College of Agriculture
Department of Horticulture • Lexington, Kentucky 40546
times the label rate, approximately 1 Tbs/gal. and 3 Tbs/gal., respectively. EcoNutrients, 0.2N0.4P-0.8K (21 percent digested bull kelp, Nereocystis luetkeana, from Northern California) was
applied at the label rate, 2 Tbs/gal. Omega 6N-2.6P-5K (microbe-digested organic fertilizer
derived from blood meal, bone meal, and sulfate of potash) was applied at the label rate and onehalf the label rate, approximately 2 Tbs/gal. and 1 Tbs/gal., respectively.
Three crops of lettuce were grown sequentially, one in September, the second in October,
and the third in November of 2000.
Cups were filled with a peat-based
germination media (Scott's Redi-Earth,
Marysville, Ohio) and placed in trays.
Bibb lettuce ‘Ostinata’ and Grand
Rapids lettuce ‘Red Sails’ seeds were
sown in the cups and germinated at an
average daily temperature of 76°F.
Seedlings were grown for 14 days and
fertigated with 150 ppm 20-10-20
inorganic fertilizer before placement in
the hydroponic tanks. The plants grew
under natural light conditions, and the
greenhouse had a heat set point of
60°F and a ventilation set point of
75°F.
Plants were harvested from the tanks after 30 days, and the fresh and dry weights were
measured. Five water samples were taken during each crop and analyzed as standard greenhouse
water samples (available nutrients) and as organic samples (total nutrients). The September crop
evaluated the label rate of Algamin, EcoNutrients, and inorganic fertilizer; the October crop
evaluated the 3X label rate of Algamin, the label rate of Omega, and inorganic fertilizer; and the
November crop evaluated inorganic fertilizer, the one-half label rate, and the label rate of Omega
in randomized complete block experiments.
Results and Discussion
Water soluble materials derived from algae (Algamin and EcoNutrients) had little value as
an organic fertilizer for lettuce in a tank hydroponic system. Dry weight of lettuce grown with
these materials was only 10 to 18 percent of those grown in inorganic fertilizer, depending on the
cultivar (Table 1). Nitrate nitrogen and phosphorus levels were less than 1 percent, and potassium
was 5 to 20 percent of the recommended levels when used at the label rate or the 3X label rate for
these fertilizers (Table 2). These results are comparable to our previous unpublished trials with
fish waste (fish emulsion and fish powder). However, the poor plant growth with fish waste was
attributed to the high biological oxygen demand from the fertilizers that prevented root penetration
into the nutrient solution for three or more weeks, despite moderate nutrient levels.
Dry weight of lettuce grown with a formulated organic fertilizer (Omega) was similar or
significantly lower than lettuce grown in inorganic fertilizer, depending on the cultivar (Table 1).
Although dry weights were similar, head size was visually smaller with the organic fertilizer.
Nitrate levels were 50 percent, P levels were 300 percent, and K levels were 100 to 120 percent of
recommended levels (Table 2).
Production of lettuce in this study was simplified when compared to the sophisticated
practices evaluated by Thompson et al. (1998). A commercial fertilizer was used as a control,
rather than a formulated fertilizer. Plus, the inorganic fertilizer was supplemented only with
additional fertilizer as the conductivity decreased and pH was not manipulated. The pH dropped
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dramatically throughout this study (Table 2), yet no apparent effects on lettuce growth were noted.
Additionally, the commercial fertilizer did not match recommended nutrient amounts precisely.
Fresh weights in this study did not reach the commercial goal of 5 ounces per head (Thompson et
al., 1998). The heads of lettuce grown in the inorganic and Omega treatments averaged
approximately 3.9 ounces. The dry weights, however, were generally similar to dry weights
reported by Thompson et al. (1998), but difficult to compare because of different temperatures and
light levels used in these studies.
In conclusion, this study indicates that it may be possible to formulate an organic fertilizer
for the hydroponic production of lettuce in the greenhouse. It is unknown if state and federal
agencies would certify such production practices as organic production.
Table 1. Mean shoot dry weight (oz) of ‘Ostinata’ (O) and ‘Red Sails’ (RS) lettuce grown with
inorganic and selected commercial organic fertilizers in 2000.
September Crop
Fertilizer
Rate
O
1
Inorganic
0.134 a
Omega
½x
Omega
1x
Algamin
Algamin
1x
3x
0.027 b
October Crop
November Crop
RS
O
RS
O
RS
0.141 a
0.067 a
0.049 a
0.067 a
0.061 a
0.062 b
0.058 a
0.063 b
0.057 a
0.067 a
0.039 b
0.005 b
0.015 b
0.049 b
EcoNutrients
1x
0.034 b
0.048 b
1 - Means followed by the same letter in a column are not significantly different at p = 0.05 according to
the Least Squares Means procedure.
Table 2. Recommended amounts (Muckle, 1993; Thompson, 1998) and measured amounts of
macronutrients and pH in inorganic and selected commercial organic fertilizers used during
September, October and/or November crops of lettuce in a pond culture system.
Fertilizer type
Recommended
amounts
Inorganic
Omega
Omega
Algamin
Algamin
EcoNutrients
20-4-16.6
6-2.6-5
1x
6-2.6-5
½x
.2-0-3.3
3x
.2-0-3.3
1x
.2-.4-.8
1x
12
8
4
4
4
4
N-P-K
Rate
Weeks
Used
NO3 ppm
125-156
90
80
42
2
0
0
NH3 ppm
0
6
22
11
0
0
0
P ppm
28-31
34
95
45
2
0.3
0
K ppm
215-252
140
Ca ppm
84-93
240
145
48
8
10
10
x
0
0
16
8
6
x
0
0
70
21
4
Mg ppm
24-26
10
pH
5.6-6.0
4.8
5.8
6.5
7.1
7.4
7.5
1.2
1.2
2.0
1.4
0.9
0.3
0.1
EC dSm-1
3
x - Municipal water used for the nutrient solutions added a mean of 52 ppm Ca and 27 ppm Mg and
negligible amounts of N, P, and K.
Literature Cited
Muckle, M.E. 1993. Hydroponic Nutrients. Growers Press, Inc., Princeton, B.C., Canada.
Thompson, H.C., R.W. Langhans, A.J. Both, L.D. Albright. 1998. Shoot and root temperature
effects on lettuce growth in a floating hydroponic system. J. Amer. Soc. Hort. Sci. 123(3):361364.
Thompson, G. 2000. International consumer demand for organic foods. HortTechnology
10(4):663-674.
For More Information:
A Handbook for the Production of CEA-Grown Hydroponic Lettuce—Cornell University
<http://www.cals.cornell.edu/dept/flori/lettuce/index.html>
Hydroponic Food Production. A Definitive Guide to Soilless Culture. 1994. Howard M. Resh. 5th
ed. Woodbridge Press, Santa Barbara, Calif.
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